Echo check circuit

ABSTRACT

A non-impact computer output printer is disclosed wherein characters distributed about a rotating drum are projected to a xerographic drum by means including a row of flash lamps located inside the character drum. The echo check circuit is used for informing the controller unit issuing the commands to the flash lamps whether or not the lamps fired. In this way the integrity of the resultant printed matter is controlled. The echo check circuit includes noise isolating means for operating in the electrically noisy environment associated with flash lamps.

0 United States Patent 11 1 [1 11 3,816,727 McCann June 11, 1974 ECHO CHECK CIRCUIT 3,560,926 2/1971 Mrkvicka 235/153 AS [75] In ento Ja es J- M c Pittsford, NY. 3,705,543 12/1972 Rees 95/4.5 R gn erox p n, S m r Primary Examiner-Charles E. Atkinson onn.

[22] Filed: Nov. 24, 1972 [57] ABSTRACT [2!] Appl. No.: 309,292 A non-impact computer output printer is disclosed wherein characters distributed about a rotating drum are projected to a xerographic drum by means includi" 235/153 95/4'5 ing a row of flash lamps located inside the character [58] Fieid 101/93 drum. The echo check circuit is used for informing the 95/4 5 controller unit issuing the commands to the flash I lamps whether or not the lamps fired. In this way the tegrity of the resultant printed matter is controlled. [56] References C1ted 1F he echo check circuit 1ncludes no1se isolat ng means UNITED STATES PATENTS for operating in the electrically noisy environment as- 3,l67,002 1/965 Kaneda 6! 3| 101/93 R ociated with l3mps 3,246,292 4/1966 Woo 235/153 AS 7 3,262,379 7/1966 Bauer, Jr. et al. 95/45 R 9 Claims, 5 Drawing Figures 45 50 I 26. j l ,1! 53' 1t& l 255,, 311.: t

=1 J2 /2p.s I .1, g l- PATENTEDJUN 1 1 mm 3.818; 721' SHEET 1 UF 5 FIG.

PATENTEB JIIII 1 I814 SHEET 2 UF 5 PATENTEB JUN I I i974 SHEET 3 OF 5 ECHO CHECK CIRCUIT BACKGROUND OF THE INVENTION This invention relates to non-impact computer output printers and the like. More specifically, the present invention relates to an echo check method and apparatus for monitoring the firing, flashing or igniting of a gas discharge lamp used to effect the impactless printing of a character.

The integrity of the printed matter generated by a computer output printer is significantly important. A computer user customarily demands I percent accuracy of the print out of computer stored information or its equivalent because errors could have very unfavorable economic, social and legal repercussions. Impactless printers of the type employing flash lamps to construct a character encounter printing errors when the lamp misfires by either flashing when not requested or not flashing when commanded to do so. When a misfire occurs, the printer controller should be alerted immediately so that the character line or page being printed can be aborted or reprinted. The echo check circuit of this invention provides method and apparatus for detecting the firing of a gas discharge flash lamp in re sponse to a controller issued flash signal and for generating an echo check signal which is used by the controller to respond appropriately to a lamp misfire.

An echo check mechanism for impactless printers of the above type should be highly reliable. Quite obviously, the echo check circuit itself can give rise to printing errors or needless printing shutdowns if it fails to detect a lamp flash or erroneously reports an ignition. Reliability is not easily come by because gas discharge lamps are notoriously electrically noisy. Hence, if a plurality of these flash lamps are rapidly firing, the echo check circuit must consistently distinguish"noise from an actual firing.

Another object-of the invention is to devise method and apparatus for isolating a circuit for detecting lamp ignitions from electrical noise.

Yet another object of the present invention is to generate an echo check signal indicative of a lamp firing within the time frame provided between successive character intervals.

Even a further object of this invention is to devise an echo check circuit suitable for an impactless printer of the type disclosed in US. Pat. No. 3,677,148 issued July I8, 1972 to Philip L. Chen. The disclosure of that patent is hereby expressly incorporated herein.

The printer described in the Chen patent includes a character drum rotated past a row of flash lamps positioned within the volume of the drum. The drum has four character sets or fonts scribed about its periphery which comprise a group. There are 33 groups arranged longitudinally to the drum axis and an equal number-of lamps. This arrangement permits a row of 132 characters to be printed. Each lamp results in the printing of four of those characters. Thefour character sets comprising a group are distributed in four separate sectors around the periphery of the drum with each character set being aligned opposite one of the 132 possible character positions forming a row. Therefore, the fastest firing case encountered by an individual lamp is when it must print all four character positions under its control within one revolution of the character drum. In other words, the fastest firing case for a lamp is four flashes per one revolution of the character drum. The characters on the character drum are illuminated by the flash lamp and projected to a photosensitive drum rotating at a substantially slower angular velocity by an appropriate lens system. The electrostatic image on the photosensitive drum is rendered visible by microscopic toner particles which in turn are electrostatically trans ferred to plain paper yielding a permanent record of the printed characters.

The above and other objects of the instant invention are accomplished with a novel echo check circuit which is coupled to the lamp ignition circuit through an isolation transformer. The echo circuit includes an integrating amplifier or ramp generator that is switched into operation the instant the firing charge of a storage capacitor is dumped into a flash lamp. The output of the ramp generator is fed to a NAND gate whose output constitutes the echo check signal sent back to the controller..The duration of the echo check signal is determined by the slope of the ramp signal and the duration is selected to insure that it does not overlap the timing pulse indicative of a subsequent character in the previously discussed character set. The other signal to the NAND gate is a pulse used to switch the ramp generator into operation and is of a length substantially equal to the time for the gases in the flash lamp to revert to a low if not non-excitation state, i.e., to become deionized.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects of the present invention will be apparent from the present description and from the drawings which are:

FIG. 1 is a perspective view of a non-impact computer output printer employing a row of flash lamps located inside a character drum wherein characters illuminated by the flash lamps are projected by a lens system to a photosensitive surface.

FIG. 2 is a block diagram representation of an electrical controller unit designed to operate the selective flashing of the lamps shown in FIG. 1 to compose a line of characters conveying a meaningful message.

FIG. 3 is a plan view of the periphery of the character drum of FIG. 1 unwrapped and laid out flat to depict the arrangement of the timing slots and character sets and groups.

FIGS. 4A and 4B are the circuit diagrams of the present echo check circuit.

DETAILED DESCRIPTION The non-impact printer shown in FIG. 1 is shown in skeleton form to make the present discussion clearer. For more details, the reader is referred to the abovenoted Chen US. Pat. No. 3,677,148. The character drum 1 includes a support cylinder 2 which has a photographic film sheet 3 wrapped about its periphery. The characters, character sets and character groups are laid out on the film sheet in the manner depicted in FIG. 3 which will be discussed shortly.

The pluralityof flash lamps 4 are positioned within the volume of drum 1 in a staggered row arrangement and the reader is referred once again to the Chen patent for more details on this alignment. It is enough for the clear presentation of this discussion to point out that each lamp is aligned to illuminate the characters in the four character sets comprising a character group.

A plurality of lens singlets 6 are disposed between drum 1 and xerographic or photosensitive drum 7 so as to project an image of the characters illuminated by a lamp to drum 7. There is one singlet for each lamp. The photosensitive drum includes a photoconductive layer 8 over a grounded conductive cylindrical core 9. The free surface of the photoconductive layer has electrostatic charge deposited onto it prior to exposure to the projected characters. The exposing light image of a character creates a latent electrostatic image which is rendered visible by microscopic, charged toner particles brought adjacent the latent image. The visible or developed image is thereafter transferred to plain paper and heated or otherwise fused'to the paper to yield a permanent record of the computer composed message. Clearly, the reader may refer to the Chen patent for more details on the described process and apparatus.

From the above, it is apparent that a page, e.g., 64 lines, of characters can be printed by rotating drum 1 past the flash lamps 4 at an angular velocity sufficiently greater than that of the photosensitive drum such that a line of characters can be printed before drum 7 rotates through any substantial angular displacement.

, The line of characters is composed by a controller unit such as a digital computer by issuing lamp ignition commands to the proper flash lamps at the proper time.

To understand the controller operation it is helpful to examine the layout of the film sheet 3. Referring to FIG. 3, a plurality of character sets 16 composed of n characters 13 are arranged in parallel along the longitudinal axis of the drum as indicated by arrow 17. (The circumference of. the drum is indicated by arrow 18.

- That is, the film sheet dimension indicated by arrow 18 is the one wrapped about the circumference of the drum.)

A character group includes the four character sets 16 found within dimension 20, e.g., sets l6A-16D. Each set is located in a different sector or quadrant of the drum circumference and each is offset some longitudinal distance from the other sets in the group. Consequently, as the drum 1 rotates the four sets l6A-16D are sequentially positioned in the path of the projection system comprising the lamp 4 and lens singlets 6 assigned to that group. Each set within the group is the character font or source for one of the four contiguous character positions in the line of characters to be printed.

The timing slots 12 are distributed about the circumference of drum 1 with each timing slot positioned opposite a character in a set. The last timing slot in sets such as sets 16A, B and C are of a greater width than the other slots to indicate the end of a set. The spacing 21 between sets is selected to provide time for the gases in a lamp 4 to fall to a low or non-excitation state before the next firing. The last timing slot in sets such as set 16D is even greater than the other last timing slots to indicate to the controller which quadrant, sector or set is coming into the path of the projection system composed of a lamp 4 and lens singlet 6. The lamp 14 is positioned inside drum 1 to illuminate the timing slots 12. The photodiode 15 is positioned to detect the light passing through a timing slot and generates a timing signal or pulse fed to the controller.

The echo check circuit of FIG. 4 may now be conveniently described. The circuiting of FIG. 4 exists for each lamp. For example, in the Chen patent there are 33 lamps each providing four character positions yielding a total line length of 132 characters. The controller logic is illustrated by the block diagram of FIG. 2 which will be discussed more fully elsewhere. For now, it suffices to say that the computer memory and processor defining the page of characters to be printed is represented by page buffer 25. The page information is gated character by character out of the buffer 35 with a lamp flash command signal 26 being issued once each sector time, i.e. the time for one sector or character set 16 to be rotated on drum 1 past a lamp 4 and lens 6.

The flash command 26 is a 4 volt, 3 micro (1.1.) sec pulse which is detected by the impedence matching portion of circuit 30. Circuit 30 includes the transistor switch 31 having the isolation transformer 32 coupled to its collector. The base-emitter junction of transistor 31 is reverse biased for noise rejection. Transformer 32 provides a 2 to l voltage gain to yield the 10 volt 3 u sec pulse 33.

The secondary of transformer 32 is coupled to tran-' sistor 34 which comprises an emitter follower switch which in turn is coupled to the gate of silicon control rectifier SCR 35. The flash command generated signal 33 turns on the emitter follower 34 allowing the charge on capacitor 38 to discharge into the gate of SCR 35. The emitter-base junction of transistor 34 and the gate cathode junction of SCR are also reversed bias for noise rejection.

Firing of the SCR 35 couples the positively charged terminal of capacitor 39 to ground. This causes the capacitor 39 to discharge into the primary of transformer 40. The inductor 41 isolates the power supply 42 to which it is coupled during the discharge of capacitor 39. i

The voltage of the secondary of transformer rises rapidly the instant capacitor 39 begins to discharge. As it does, the sparker electrode 43 in the lamp 4 generates free ions in the lamp gas to aid the firing of the lamp. As the secondary voltage on transformer 40 continues to rise, a breakdown in the lamp gas occurs between the trigger electrode 44 and the cathode 45 followed by a complete ionization of the gas between anode 46 and cathode 45. At this time, the lamp firing capacitor 50 is discharged through the lamp to ground and the lamp flashes. The primary of transformer 51 isolates the power supply 52 during the discharge time of capacitor 50.

The secondary of transformer 51 is used to generate the controlled width echo check" pulse sent back to the controller. The voltage on the secondary is the negative pulse 53 having a duration substantially equal to the time required to saturate transformer 51. Pulse 53 cuts-off transistors 55 and 56 and turns on transistor 57. The capacitor 58 and resistor 59 along with transistor 57 comprise an operational amplifier integrator or ramp generator that produces the ramp signal 62. The ramp 62 is fed to NAND gate 63 along with the signal 64 at the collector of transistor 56. The output of the NAND gate is the echo check pulse 65 after it is inverted by gate 67. The echo check pulse is fed by the emitter follower 68 to the line driver 69. Line driver 69 transmits the echo pulse to the controller.

The controller detects a misfire by strobing the line driver 69 output at a time subsequent to the issuance of the flash pulse 26 and does so at a time around the mid point of the echo pulse. 1f the echo pulse is detected, the controller knows the lamp fired. If the echo pulse is not detected, or if it is detected but not subsequent to a flash signal 26, the page being printed is aborted, reprinted or flagged or some combination thereof according to what is desired by the user.

Following the lamp flashing process, the circuit of FIG. 4 must be readied for the next flash. SCR 35 is shut off when inductor 41 saturates, which is about one half the time for transformer 51 to saturate. Consequently, capacitor 39 recharges through resistor 71 and diode 72.

The saturation time for transformer 51 is selected to match as near as practical the deionization time required for the lamp to make it ready for the next firing. With transformer 51 saturated, capacitor 50 is recharged through resistor 74 and diode 75 until the diode becomes back biased. From that point on, capacitor 50 is charged toward the potential of power supply 76 through resistors 77 and 78 and is clamped by diode 79. The circuit is now ready to receive another flash command pulse 26.

A lamp can and may flash only once per sector (i.e. the time for one character set 16 to rotate past a lamp 4 and lens singlet 6). The shortest period between any two flashes is thatrepresented by the gap 21 between character sets within a group. (see FIG. 3) This represents the worst case where the first character in one set is flashed immediately after the last character in the previous set. The charging time of capacitor 50, the energy storage capacitor for the lamp, is matched to about one half the time corresponding to gap 21. Due to the characteristic of gas discharge lamps (specifically lamp number 210394, manufactured by E. G. &

' (3., Salem, Mass.) no voltage may be placed across the lamp for about half the time corresponding to gap 21 to insure that the gas in the lamp completely deionizes. The time required for transformer 51 is selected to match the deionizing period required for a lamp, i.e. to be about one half the time corresponding to gap 21.

The block diagram of FIG. 2 illustrates, in general, the cycle of operation for firing of the flash lamps in accordance with the output from a computer 25 which is utilized as a buffer storage device containing sufficient data to print a complete page of 64 lines. A model DD? 516 computer manufactured by Honeywell Corporation has been satisfactorily employed to demonstrate the operation of the printer disclosed in the Chen patent supra.

Initially only a portion of the data contained in the buffer device 25 is written into a memory device 81 which may comprise a pair of FT .40 fast access memory cards having a capacityof l6 two-character words at 16 bits per word. Such cards are manufactured by Xerox Data Systems. Writing into memory is accomplished by means of a process control shift register 82 driven by a 5 MHz clock. The shift register also controls the reading of the data into a comparator 83 which may comprise suitable logic components, for example, AND gates (not shown) for deriving outputs when positive comparisons are made. The other input to the AND gates may be derived from acounter 84.

The counter 84 is preset to a code representing the first character on the character drum 1. Accordingly, when the first or initial portion of data is read into the comparator 83 it is compared to the code representing the aforementioned first character or in other words the preset value of the counter. Each successive character on the character drum is represented by a code in the counter on incrementing thereof. This incrementing of the counter is accomplished by virtue of the pulses from the timing slit sensor, photodiode 15. it can be seen that the code representing the second character on the drum will be represented by the preset value of the counter plus 1.

When the code in the comparator 83 is the same as the code from the memory 81, a latch 85, which may comprise a flip-flop (not shown) is set. Pulses from the sensor 15 are ANDed with the outputs from the latches to thereby generate the flash command pulse 26. The echo check pulse is fed back to computer 25 for processing.

The foregoing description has included specific numbers of characters per line, character sets per group and groups per line. Clearly, these are matters of design choice and may be other than that disclosed herein. Specifically, a saving in lamps is possible if each character group 20 includes six character sets 16. In this arrangement, only 22 lamps are needed to compose a line of print having up to 132 characters.

What is claimed is:

1. in a non-impact character printer of the type employing a plurality of flash lamps to illuminate selected characters from character sets moved relative to the flash lamps, the flash lamps being illuminated in response to flash command signals generated by controller means in a sequence controlled by timing pulses associated with the characters in the character sets. the flash signals being supplied to lamp ignition means coupled to the controller means and the flash lamps, the improvement comprising apparatus for generating an echo check signal indicating a lamp has flashed includmg:

integrating means coupled to said lamp ignition means for generating at the time of a lamp flash, a first signal changing in amplitude between two limits within a time that determines the duration of the echo check signal, and

gate means coupled to receive said first signal and to generate an echo check signal indicative of a lamp flash having a duration corresponding to the time required for the first signal to change amplitude from one to another of its two limits.

2. The apparatus of claim 1 wherein said controller means is coupled to receive said echo check signal for performing a decision function when an echo check signal is not received subsequent to issuance of a flash command signal or an echo check signal is received when a flash command signal is not issued by said controller means.

3. The apparatus of claim 1 further including pulse generating means coupled between said lamp ignition means and said gate means for generating a second signal of fixed amplitude that is logically ANDed with said first signal to isolate further the echo check apparatus from electrical noise including that associated with a lamp flash.

4. The apparatus of claim 1 further including an isolation transformer having primary windings coupled between a power supply and the said lamp ignition means to protect the power supply during lamp flash and secondary windings coupled to said integration means to isolate further the integrating means from electrical noise including that associated with a lamp flash.

5. The apparatus of claim 4 wherein said isolation transformer is a saturable transformer having a saturation period corresponding to the time required for the gases in a flash lamp to deionize to a state in which the lamp can again be flashed.

6. The apparatus of claim 5 wherein said saturation period of the transformer is a multiple of the time spacing between characters in a character set and the duration of the echo check pulse is less than the intercharacter spacing. V

7. The apparatus of claim 1 wherein said integrating means includes a ramp generator producing a ramp signal as said first signal.

8. The apparatus of claim 7 further including a saturable transformer having its primary windings coupled between said lamp ignition circuit and a power supply and its secondary windings coupled to a pulse generating circuit and said ramp generator and wherein said pulse generator produces a second signal of fixed amplitude and is coupled to the gate means so the second signal is logically ANDed with said ramp signal for the production of said echo check signal.

9. In a non-impact character printer of the type employing a plurality of flash lamps to illuminate selected characters from character sets in response to flash command signals generated bycontroller means moved relative to the lamps in a sequence controlled by timing pulses associated with the characters in the characters sets, the improvement comprising the method of generating an echo check signal for informing the controller unit that a lamp has flashed including generating a first signal changing in amplitude between first and second limits within a time that determines the duration of the echo check signal in response to a flash of a lamp generating a second signal of fixed amplitude at the same time as said first signal, and

comparing said first and second signals and generating said echo check signal during the time said first signal changes between said limits whereby said echo check signal is isolated from electrical noise including that associated with the flashing of said lamps. 

1. In a non-impact Character printer of the type employing a plurality of flash lamps to illuminate selected characters from character sets moved relative to the flash lamps, the flash lamps being illuminated in response to flash command signals generated by controller means in a sequence controlled by timing pulses associated with the characters in the character sets. the flash signals being supplied to lamp ignition means coupled to the controller means and the flash lamps, the improvement comprising apparatus for generating an echo check signal indicating a lamp has flashed including: integrating means coupled to said lamp ignition means for generating at the time of a lamp flash, a first signal changing in amplitude between two limits within a time that determines the duration of the echo check signal, and gate means coupled to receive said first signal and to generate an echo check signal indicative of a lamp flash having a duration corresponding to the time required for the first signal to change amplitude from one to another of its two limits.
 2. The apparatus of claim 1 wherein said controller means is coupled to receive said echo check signal for performing a decision function when an echo check signal is not received subsequent to issuance of a flash command signal or an echo check signal is received when a flash command signal is not issued by said controller means.
 3. The apparatus of claim 1 further including pulse generating means coupled between said lamp ignition means and said gate means for generating a second signal of fixed amplitude that is logically ANDed with said first signal to isolate further the echo check apparatus from electrical noise including that associated with a lamp flash.
 4. The apparatus of claim 1 further including an isolation transformer having primary windings coupled between a power supply and the said lamp ignition means to protect the power supply during lamp flash and secondary windings coupled to said integration means to isolate further the integrating means from electrical noise including that associated with a lamp flash.
 5. The apparatus of claim 4 wherein said isolation transformer is a saturable transformer having a saturation period corresponding to the time required for the gases in a flash lamp to deionize to a state in which the lamp can again be flashed.
 6. The apparatus of claim 5 wherein said saturation period of the transformer is a multiple of the time spacing between characters in a character set and the duration of the echo check pulse is less than the inter-character spacing.
 7. The apparatus of claim 1 wherein said integrating means includes a ramp generator producing a ramp signal as said first signal.
 8. The apparatus of claim 7 further including a saturable transformer having its primary windings coupled between said lamp ignition circuit and a power supply and its secondary windings coupled to a pulse generating circuit and said ramp generator and wherein said pulse generator produces a second signal of fixed amplitude and is coupled to the gate means so the second signal is logically ANDed with said ramp signal for the production of said echo check signal.
 9. In a non-impact character printer of the type employing a plurality of flash lamps to illuminate selected characters from character sets in response to flash command signals generated by controller means moved relative to the lamps in a sequence controlled by timing pulses associated with the characters in the characters sets, the improvement comprising the method of generating an echo check signal for informing the controller unit that a lamp has flashed including generating a first signal changing in amplitude between first and second limits within a time that determines the duration of the echo check signal in response to a flash of a lamp generating a second signal of fixed amplitude at the same time as said first signal, and comparing said first and second signals and generating said echo check sigNal during the time said first signal changes between said limits whereby said echo check signal is isolated from electrical noise including that associated with the flashing of said lamps. 